Recently, a GaN-based light emitting diode (LED) has been widely used as a light emitting device. GaN-based LEDs have significantly advanced the LED technology and have been employed in various applications including full color LED displays, LED traffic signals, white LEDs and the like.
Recently, high-efficiency white LEDs have been expected to replace fluorescent lamps. In particular, the efficiency of white LEDs is approaching that of ordinary fluorescent lamps. However, the LED efficiency can be further improved, thus its continuous improvement will be required.
Two major approaches to improve the efficiency of LEDs have been attempted. The first approach is to enhance the internal quantum efficiency determined by the crystal quality and the epitaxial layer structure, and the second one is to increase the light extraction efficiency. Since the internal quantum efficiency currently reaches 70˜80%, there is little room for further improvement of the internal quantum efficiency. However, the light extraction efficiency may be further improved. The technology for improving the light extraction efficiency mainly relates to the enlargement of the “escape cone.” However, the enlargement of the escape cone cannot completely eliminate light loss due to the total internal reflection. Further, even for light radiated into the escape cone, reflection loss occurs due to the refractive index mismatch between the LED and the surrounding media.
A new approach to reduce total internal reflection and reflection loss and improve light extraction efficiency is disclosed in U.S. Pat. No. 5,955,749, entitled “Light emitting device
utilizing a periodic dielectric structure” by Joannopoulos et al. According to the U.S. Pat. No. 5,955,749, a lattice of holes is formed in semiconductor layers of a light emitting diode to create photonic crystals. The lattice creates a medium with a periodically varying dielectric constant and affects a path of light transmitting through the medium. This lattice forms a photonic band gap (PBG) and photons having energy within the PBG cannot propagate inside the photonic crystal. Therefore, if a photonic crystal is formed such that the energy of photons emitted by LED is within the PBG, the lateral propagation of photons is prohibited to allow all the photons to be exited from the LED to the outside, so that the light extraction efficiency can be improved.
However, the aforementioned U.S. Pat. No. 5,955,749 discloses an LED having GaAs-based n-type, p-type and active layers, and the lattice of holes is formed on these layers. Thus, during the formation of the holes by etching the GaAs-based layers, the layers can be easily damaged. In addition, GaAs has a very high surface recombination rate at which holes and electrons are recombined in a surface thereof. The surface recombination of holes and electrons does not generate light having a desired wavelength. Thus, the light quantum efficiency is seriously reduced, since the surface area of the active layer is increased as the lattice of holes is formed.
Furthermore, a light emitting diode is a semiconductor device which is formed into a p-n junction structure of semiconductors and emits light by the recombination of electrons and holes, and the light emitting diode is generally driven by current flowing in one direction. Thus, in a case where the light emitting diode is driven by alternating current, an A/D converter is required to convert AC into DC. As the A/D converter is utilized together with a light emitting diode, the installation cost of the light emitting diode is increased. In particular, it is difficult to utilize a light emitting diode as general household illumination. In order to replace the existing fluorescent lamps with LEDs, there is a need for a novel light emitting diode which can be driven using an AC power source without an A/D converter.
The present invention is conceived to solve the aforementioned problems in the prior art. An object of the present invention is to provide a light emitting device of which light extraction efficiency is improved by employing a photonic crystal structure and which can be directly driven using an AC power source without an A/D converter.
According to an aspect of the present invention for achieving the object, there is provided an AC light emitting device having photonic crystal structures. The light emitting device includes a plurality of light emitting cells and metallic wirings for electrically connecting the light emitting cells with one another. Further, each of the light emitting cells includes a first conductive type semiconductor layer, a second conductive type semiconductor layer disposed on one region of the first conductive type semiconductor layer, and an active layer interposed between the first and second conductive type semiconductor layers. In addition, a photonic crystal structure is formed in the second conductive type semiconductor layer. The photonic crystal structure prevents light emitted from the active layer from laterally propagating, such that light extraction efficiency of the light emitting device can be improved. Furthermore, the metallic wirings electrically connect a plurality of light emitting cells with one another such that an AC operated light emitting device can be provided.
The photonic crystal structure may have various shapes and be regularly arranged two-dimensionally. For example, the photonic crystal structure may include a lattice of holes which are formed in the second conductive type semiconductor layer and arranged two-dimensionally. Alternatively, the photonic crystal structure may include a periodic unevenness formed by partially etching the second conductive type semiconductor layer or a lattice of second conductive type semiconductor rods formed by etching the second conductive type semiconductor layer. The photonic crystal structures are periodically arranged to create a photonic band gap and also prevent light from laterally propagating. Thus, the light emitted from the active layer cannot be laterally propagated by means of the photonic crystal structure and can thus be exited to the outside. Accordingly, light extraction efficiency of the light emitting device can be improved.
Furthermore, first electrode pads may be formed in other regions of the first conductive type semiconductor layers. In addition, transparent electrodes cover the second conductive type semiconductor layers. Moreover, second electrode pads may be disposed on the transparent electrodes. In this case, each of the metallic wirings connects the first and second electrode pads of the adjacent light emitting cells.
According to another aspect of the present invention for achieving the object, there is provided a method of fabricating an AC light emitting device having photonic crystal structures. The method of the present invention comprises forming a first conductive type semiconductor layer, an active layer and a second conductive type semiconductor layer on a substrate. The second conductive type semiconductor layer, the active layer and the first conductive type semiconductor layer are patterned to form a plurality of light emitting cells. Each of the light emitting cells includes an isolated first conductive type semiconductor layer, a second conductive type semiconductor layer disposed on one region of the isolated first conductive type semiconductor layer, an active layer interposed between the isolated first conductive type semiconductor layer and the second conductive type semiconductor layer, and a photonic crystal structure formed in the second conductive type semiconductor layer. Metallic wirings for electrically connecting the light emitting cells having the photonic crystal structure are then formed. According to the present invention, since the photonic crystal structure is formed in the second conductive type semiconductor layer, a fabricating process can be simplified. Further, since the active layer and the first conductive type semiconductor layer need not be etched, the etching time can be shortened. Accordingly, etching damages which may occur in the first conductive type semiconductor layer, the active layer and the second conductive type semiconductor layer can also be reduced. In addition, since the metallic wirings electrically connect the light emitting cells with one another, an AC operated light emitting device can be provided.
The photonic crystal structure may be formed by etching the second conductive type semiconductor layer using photolithographic and etching processes, and an e-beam lithography or hologram technique can be used.
Further, transparent electrodes are formed on the second conductive type semiconductor layers having the photonic crystal structure. In addition, first electrode pads may be formed on other regions of the first conductive type semiconductor layers, and second electrode pads may be formed on the transparent electrodes. Each of the metallic wirings connects the first and second electrode pads of the adjacent light emitting cells. According to the present invention, an AC light emitting device having photonic crystal structures can be provided, in which light extraction efficiency can be significantly increased.